Multicarrier Principles
A multi carrier (MC) arrangement with frequency division duplex (FDD) can be described as a set of downlink carriers linked to a set of uplink carriers. The downlink carriers can be adjacent or non-adjacent in the frequency domain, and the same holds for the uplink carriers. Multi-carrier arrangements can also be used in time division duplex (TDD) systems. The component carriers in a multi-carrier system may also belong to different frequency bands. The primary objective of the multi-carrier system is to achieve higher data rates in the downlink or in the uplink or in both directions.
Operation of Wideband Code Division Multiple Access/High Speed Packet Access (WCDMA/HSPA) on multiple of 5 MHz carrier frequencies is in fact an evolution of WCDMA and HSPA. This mode of operation is often referred to as Multi Carrier WCDMA or Multi Carrier HSPA. Similar evolution is taking place in Evolved Universal Mobile Telecommunications System Terrestrial Radio Access Network (E-UTRAN) system, where multiple component carriers (e.g. 4×20 MHz in DL and 2×20 MHz in UL for FDD) shall be used to enhance the data rate.
Generally, in multi-carrier systems more than one carrier is used at least in the downlink or the uplink. One of the multi carriers is called anchor carrier (a.k.a. primary carrier) and remaining ones are called secondary carriers (a.k.a. supplementary carriers).
The anchor carrier (alternatively referred to as primary carrier) contains all types of physical channels including all common control channels. The secondary carriers may or may not contain all types of physical channels; for instance they may lack some of the common downlink control channels. The anchor carrier in downlink and in uplink (i.e. if there is more than one carrier in uplink) should work according to the legacy operation. Note that legacy operation is based on a single carrier. This means that the downlink anchor carrier should contain all common channels. This is to make sure that firstly the legacy single-carrier capable User Equipments (UEs) are served. Secondly also the multi-carrier UE requires that at least the anchor carrier transmits all common control channels for acquisition of the frame timing, neighbor cell measurements etc.
In short, the downlink anchor carrier of a multi-carrier system includes the same types of physical channels as the downlink carrier in a single carrier system, while a secondary downlink carrier of the multi-carrier system may be missing some of the types of channels which are included in the anchor carrier.
For instance a UE in dual cell High Speed Downlink Packet Access, HSDPA, (DC-HSDPA) operation, which is specified in release 8, is able to simultaneously receive HSDPA traffic over two downlink carrier frequencies, see third generation partnership project technical specification 3GPP TS 25.214, “Physical layer procedures (FDD)”.
They are also transmitted in the same frequency band from a single serving sector. There is one uplink carrier for a DC-HSDPA UE and it is not strictly tied to one of the two downlink carriers. In DC-HSDPA UE, the anchor carrier has all types of physical channels including Fractional Dedicated Physical Channel (F-DPCH), E-DCH HARQ Acknowledgement Indicator Channel (E-HICH), Enhanced Absolute Grant Channel (E-AGCH), and E-DCH Relative Grant Channel (E-RGCH). During dual carrier operation in CELL_DCH, one of the downlink carriers is the UE secondary carrier, which is not the UE anchor carrier, see third generation partnership project technical specification 3GPP TS 25.214, “Physical layer procedures (FDD)”.
Any system can be evolved to a multi-carrier system since it leads to many-fold increase in data rate. The future advancements of HSPA, E-UTRAN and other systems would culminate into a multi-carrier system with multiple carriers both in uplink and downlink (e.g. 4 downlink carriers and 2 uplink carriers). There is also an ongoing work to introduce dual carrier for uplink under the work item called dual cell HSUPA. In case of dual cell HSUPA operation, there are at least 2 downlink carriers.
UE Advanced Receiver Capabilities
In release 5 the UE receiver performance requirements are solely based on the baseline classical rake receiver. The corresponding requirements are commonly termed and specified as the minimum performance requirements in third generation partnership project technical specification 3GPP TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”.
In release 6 and beyond, enhanced UE receiver performance requirements are also specified in 3GPP TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”. In order to fulfill these requirements and pass the corresponding conformance tests, the UE will have to implement an advanced receiver e.g. receiver diversity, chip level equalizer, generalized rake receivers (G-RAKE) or similar receiver structures. A goal of the specification of the enhanced requirements is to significantly boost the downlink bit rate. In WCDMA terminology UE receiver performance requirements for various advanced receivers are specified as enhanced receiver type 1 (receiver diversity), enhanced receiver type 2 (chip level equalizer), enhanced receiver type 3 (combined receiver diversity and equalizer) and type 3i (combined receiver diversity and inter-cell interference cancellation receiver) until now. Furthermore the enhanced receiver performance specification does not preclude the UE vendors to implement advanced receivers beyond the specified enhanced requirements.
It should be noted that enhanced performance requirements are not only confined to HSDPA. In fact enhanced requirements are prevalent for a number of reception scenarios where the UE receives downlink transmissions from a Universal Terrestrial Radio Access Network (UTRAN): Dedicated Channel (DCH), Multimedia Broadcast and Multicast Service (MBMS), Enhanced Dedicated Channel (E-DCH) downlink channels etc. However, in the present description the focus is on HSDPA reception scenario and more specifically on the multi-carrier HSDPA reception scenario.
For Long Term Evolution (LTE), where a UE receives downlink transmissions from an E-UTRAN, options for interference cancellation can also be envisioned, some of which requires that the UE can synchronize and process the reference signal in order to characterize the interference. The teachings of the instant description can also be applied for this case.
Interference Cancellation Ability of Receivers
Different types of receivers lead to different levels of performance gain. The performance gain is achieved by eliminating or at least mitigating different types of interference. There are various sources and forms of interference e.g. intra-cell, inter-cell, inter-stream interferences etc. For instance in a Code Division Multiple Access (CDMA) system, such as in WCDMA, the intra-cell interference is common due to a loss of orthogonality between the channelization codes as these pass through a multipath fading channel.
On the other hand, the inter-cell interference exists in single or tight reuse systems such as in CDMA and Orthogonal Frequency-Division Multiple Access (OFDMA) based LTE system. The source of inter-cell interference is the interference from a certain number of neighbor cells.
Multiple Input Multiple Output (MIMO) transmission comprises of two ore more streams of data. MIMO provides system improvement at high Signal to Interference-plus-Noise Ratio (SINR). But the multi-stream transmission also leads to inter-stream interference, which could be eliminated or considerably reduced by using an appropriate receiver.
The enhanced receiver type 3 (a.k.a G-RAKE 2 receiver in the symbol-level implementation) is capable of eliminating intra-cell interference leading to significant performance gain over the classical rake receiver. Similarly the enhanced receiver type 3i (a.k.a G-RAKE 2+ receiver in the symbol-level implementation) is capable of eliminating both intra-cell and inter-cell interferences leading to significant performance gain over the classical rake receiver. There are several different ways to implement the type 3i receiver functionality, where two of the methods can be called parametric and non parametric solutions. The non-parametric solution estimates the net interference effect, bundling all intra- and inter-cell interference sources into one package, and tries to reduce them simultaneously. The parametric solution on the other hand attempts to explicitly model the interference as a sum of its different contributing parts. The parametric solution needs to be capable of detecting a certain number of interfering cells to suppress (ideally the strongest interfering cells). For instance the UE should identify the scrambling codes used in the strongest interfering cells and eventually use this information to eliminate or minimize the interference. The UE further needs to continuously estimate the channel response of the interfering cell(s) in order to suppress the interference. Estimating the channel response can be done in several ways, where one way is to separately estimate the power delay profile and use it to determine the channel tap delays, and separately estimate the complex weight for each of the tap delays.
It should be noted that although the discussion on interference canceling receivers so far has been limited to the by 3GPP specified Type 3i (Parametric) receiver for WCDMA/HSPA, the knowledge of the channel responses from interfering cells could be used in many other receiver structures. Examples of other receivers could e.g. be receivers that attempt to create a replica of the received interfering signal and subtract it from the desired signal, and can also include OFDMA receivers that characterizes and mitigates the effect of neighbor cell interference.
UE Implementation Aspects
In WCDMA/HSPA, the downlink common control channels include primary synchronization channel (P-SCH), which carries primary synchronization code (PSC), secondary synchronization channel (S-SCH), which carries secondary synchronization code (SSC), primary Common Pilot Channel (CPICH), which carriers scrambling code information and broadcast channel (BCH), which carriers system information comprising of master information block (MIB) and system information blocks (SIB). The P-SCH and S-SCH together are known as SCH channel.
There are in total 8192 scrambling codes available for the system to use. This set is divided into 64 primary scrambling code groups, each containing 8 primary scrambling codes. Each primary scrambling code is associated to 15 secondary scrambling codes through a one to one mapping. The UE is not required to search for secondary scrambling codes during the initial synchronization procedure.
In a typical UE implementation, the identification of slot/frame boundary, and identification of scrambling code for the serving cell is done through a phased approach, as described below:
Stage 1: Find the slot boundary using the P-SCH
Stage 2: Find the frame boundary and the scrambling code group using P-SCH and S-SCH, where each scrambling code group contains 8 primary scrambling codes.
Stage 3: Using CPICH, determine the best primary scrambling code candidate out of the 8 contained in the scrambling code group identified in Stage 2.
In multi-carrier systems the downlink anchor carrier contains all the common channels. However, depending upon the network implementation, some of the common control channels may not be transmitted on the secondary carriers; it might for example be the case that only the CPICH is transmitted on the secondary carrier. The absence of SCH channels on the secondary carrier would increase the complexity of the inter-cell interference cancellation receiver, because the UE is unable to make use of the S-SCH to identify the scrambling code group.
For LTE, there are 504 physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity is thus uniquely defined by a number in the range of 0 to 167, representing the physical-layer cell-identity group, and a number in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group, see the technical specification 3GPP TS 36.213, “Physical layer procedures”, v 8.8.0.
Similar to WCDMA/HSPA, the LTE downlink common control channels include among others the SCH channel, which carries the primary and secondary synchronization code. The primary synchronization code carries information about the physical-layer cell-identity group, while the secondary synchronization code carries information about which of the physical-layer cell identities in the physical-layer cell-identity group that is used. Using the physical-layer cell identity, the UE can derive the exact definition of the cell specific reference signals.
Similar to WCDMA/HSPA, the SCH channel is required for the UE to identify the slot and sub-frame timing. Additionally, the SCH is in LTE systems used to obtain frequency synchronization.
In contrast to the WCDMA/HSPA cell search procedure, the LTE cell-search procedure does not include a stage 3 cell search, instead it determines, slot and subframe timing, frequency synchronization and cell-identity group using the SCH only.
In order for the UE receiver to cancel the inter-cell interference, which is caused by the neighbor cells, the channel impulse responses from each of the interfering cells are to be determined. In order to be able to do this, the UE needs to have knowledge about the timing of the neighbor cells and the scrambling codes used in those neighbor cells.
In a multi-carrier system, it is up to the network implementation whether to implement all the control channels on all the secondary carriers or not. In the absence of synchronization signals on secondary carriers, the UE will have to perform extensive search to determine the scrambling codes used in the neighbor cells. This will drain UE power and slow down the synchronization process. One way to mitigate search complexity is to signal a list of scrambling codes used by the neighbor cells on their secondary carriers. However this solution comes with the cost of a significant network overhead, that is undesirable and should be avoided or mitigated.
Furthermore, it should be noted that the book-keeping of neighbour cell lists in general is a difficult and error-prone task. There is thus a need for eliminating the risk for configuring the detailed neighbour cell list incorrectly. An incorrectly configured neighbour cell list, could lead to the UE not being able to identify the strongest interfering cells correctly.
Hence there exist a need for new methods and devices providing improved configuration of cellular radio systems operated using multiple carriers.